Thermal Stability of the Mirostructure of Mg Alloys with Segregated Stacking Faults

In the last decade, Mg alloys containing long-period ordered phase (LPSO) have been attracting considerable attention as a structural material for engineering application and transportation (airplane and automobile) industry, particularly due to their excellent strength to weight ratio.<br/>The present study focuses on investigation of the thermal stability of the microstructure of the rapidly solidified ribbon-consolidated Mg-Zn-RE (rare earth) alloys. In the as solidified state, the material has a very fine grain structure with an average grain size below 1 μm. Moreover, dispersive Zn- and RE-rich stacking faults segregated in basal planes can be observed in the microstructure. The alloys are characterized by a weak basal texture with a more pronounced intensity at the (10-10) pole. Such comprehensive microstructure results in superior mechanical properties, e.g. the Mg_97.94Zn_0.56Y_1.5 alloy has shown a yield strength of 360 MPa and an elongation of 18%. In order to study the thermal stability of such microstructures, isothermal annealing in a range of 300 °C - 500 °C was applied. Scanning electron microscopy (SEM), including backscatter electron images (BSE), and electron backscatter diffraction (EBSD) techniques have been used to study the microstructure changes. Furthermore, X-ray diffraction measurements were performed for reliable texture analysis. The microstructure was found to be stable with increasing annealing temperature up to 400 °C. With further temperature increase, the growth of the grain size and changes in the texture of the alloy, particularly, redistribution of the intensity at the (10-10) pole, can be related to the recrystallization process. Microstructure changes are correlated with the results of the microhardness measurements.